Such a control system is found in, for example, various types of work vehicle comprising a front part and a rear part, which parts each have at least one wheel axle. The vehicle parts are interconnected in an articulated manner, and the parts are rotatable relative to one another around the articulation via two hydraulic cylinders arranged between the two parts.
A loading machine, such as a wheel loader or a dumper, is an example of such a work vehicle. In this case, the first control means consists of the steering wheel of the vehicle, and the second control means consists of a lever, or joystick. The invention will be described below in a case in which the control system is arranged in a loading machine for controlling its steering cylinders. This is to be regarded in no way as being a limiting example within the area of application of the invention.
Steering in a loading machine has high safety requirements. As a rule, a loading machine has normal wheel steering, but may also in parallel be equipped with lever steering. The wheel is used during transport, and the lever is used for work at lower speeds such as, for example, for loading a truck, in order to increase driver comfort.
The control system is usually designed in such a way that the driver, by means of the wheel steering, mechanically regulates a hydraulic valve which in turn transmits hydraulic energy to the steering cylinders. The lever steering also transmits hydraulic energy to the steering cylinders, but its hydraulic valve is usually controlled by electric signals; that is to say, the driver moves the lever, which in turn generates an electric signal. In some cases, it is desirable to manipulate the electric signal from the lever (ramps, maximum levels etc.), which can be effected by virtue of the signals from the lever to the hydraulic valve being transmitted via a machine computer.
The lever steering, with its electronics, is a complex system with many sources of failure. In order to increase safety, the control system can be designed so that the wheel steering has top priority; that is to say, if the steering wheel and the lever are used at the same time, it is the wheel which prevails. If an error occurs in the lever steering (for example steering takes place in spite of the lever being in a neutral position, or if the lever has locked in a certain position), the driver can use the steering wheel and thus steer the machine in a safe way. One way of carrying out prioritization is for the machine computer to receive a signal via a sensor which senses that the driver is using the wheel, the computer thus being able to suppress the control signals going out to the hydraulic valve of the lever steering. One problem with such a system is that the quality requirements for the machine computer and other electronics are very high, which results in high costs.